Thu, 18 Dec 2008

My Diploma Thesis: Spin Transport in Mesoscopic Systems

Sometimes people ask me what I'm doing right now, and I tell them
"I'm writing my diploma thesis on mesoscopic spin transport", and they
know just as much as before. So here I want to explain what that means.

Mesoscopic systems

A mesoscopic system is one that is larger than a few nanometers, but
still small enough that you have to care about quantum effects.

That's not a very precise definition, so I'll try again: Consider a
metallic wire. For macroscopic systems (ie the ones that we are used to
in day-to-day live) you might know that the electrical resistance of
such a wire increases linearly as you increase its length, and decreases
linearly if you increase its cross section.

This is very intuitive, because electrical resistance describes how
hard it is for an electron to travel through our wire. If the wire is
longer, it sees more obstacles, so the resistance is higher. If the wire
has a larger cross section, it's easier for the electron to find a way
that's not blocked, so the resistance is smaller. That's called
Ohm's law.

These relations aren't true anymore for rather small systems. If you
have a very thin wire, say 20 nanometers, and increase its diameter by
another nanometer, the resistance might not change at all. Then you
increase its diameter by another nanometer, the resistance suddenly jumps
down by a few percent.

All these systems that are too small for Ohm's law to apply are
called mesoscopic. All mesoscopic effects have to be explained
with quantum physics, at least at some point.

Electron Spin

Electrons have something called Spin. Everybody knows that
it has a charge, and it acts as if it rotated around its own axis very
fast. So it looks like a current which runs in a circle, and that
creates a small magnetic field.

If you try to measure the magnetic field of one electron, you will
only ever get two possible values, which we call spin up and
spin down.

Spin Optics

In a semiconductor, one can split up a beam of electrons into two beams
of spin-up and spin-down electrons, just like in optics with polarized
light. That splitting can be influenced by an external voltage, like a
classical transistor.

The topic of my diploma thesis is to figure out how such spin polarized
electron beams behave in certain semiconductor systems.